Cell selection in mobile communications

ABSTRACT

Mobile user equipment for use in a cellular communications environment performs an improved cell selection on transitions out of a connected mode state. The transitions might be from one connected mode state to another or from a connected mode state to idle mode. The improvement in cell selection lies in assembling the list of candidate cells prior to cell selection. In the prior art, the list of candidate cells would usually be restricted to the serving cell or active cells supporting communication between the equipment and the network prior to transition. In embodiments of the invention, the list might comprise cells which are not the serving cell or which are outside the active set, for example a network-preferred cell or cells neighboring the serving cell or cells of the active set.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of co-pending U.S. patentapplication Ser. No. 12/773,244 filed on May 4, 2010, which is acontinuation of U.S. patent application Ser. No. 10/840,191 filed on May6, 2004, now U.S. Pat. No. 7,747,275, the contents of which areincorporated herein by reference.

The present invention relates to cell selection in mobilecommunications. It finds particular application in user equipment statetransitions, either between cell states or from a cell state to idle.

Cellular communications provides an access technology for portable userequipment (“UE”) to gain access to networks without having to use awired connection. The key is that a geographical area is divided intocells and each cell has its own wireless access point to one or morenetworks. Portable user equipment (“UE”), nowadays including suchdevices as mobile telephones, personal digital assistants and laptopcomputers, supports wireless communication as far as the access point.The one or more networks then available via the access point may beeither wireless or fixed line. The mobile UE can thus use an accesspoint from anywhere in a cell (subject to signal strength) tocommunicate with either other mobile, or with fixed line, equipment.Nowadays, a UE can also roam, moving from cell to cell while maintaininga connection, this being enabled by standardised handover protocols.

The most successful second generation (“2G”) cellular network was the“Global System for Mobile communication” (GSM). The internationalcommunity is now working towards third generation (“3G”)standardisation, based on a set of requirements drawn up by theInternational Telecommunications Union (“ITU”) known collectively asIMT-2000. IMT stands for International Mobile Telecommunications. 3Gnetworks will give users increased data rates, for example up to 2 Mbps.With such speeds, a lot of services including video, fax and internetaccess can easily be supported. However, 3G networks are not yetgenerally available and interim network solutions have become known as“2.5G” services. These include GPRS (“General Packet Radio Service”),which shares the wireless access technology dynamically between speechand data and has a theoretical maximum speed of 172.2 kbps, and EDGE(“Enhanced Data Rates for GSM Evolution”) which builds on GPRS to givepeak rates over 384 kbps.

Many mobile devices already conform to 3G standards even though 3Gnetworks are not yet generally in place. An area in which considerablework has been done is in development of the 3G UMTS (“Universal MobileTelecommunication System”) which is based on GSM technology andincorporates GPRS. The cellular access technology in this case relies onW-CDMA (“Wideband Code Division Multiple Access”). One of the groupsworking on UMTS standards is the 3^(rd) Generation Partnership Project(“3GPP”).

The UMTS network architecture usually has two primary parts, the UTRAN(“UMTS Terrestrial Radio Access Network”) and the core network. TheUTRAN looks after the physical aspects of providing wireless access fora mobile UE to the core network and the core network provides switching.The UTRAN and the core network communicate with each other via aninterface known as the “Iu”.

The UTRAN is made up of a set of RNSs (“Radio Network Subsystems”). EachRNS comprises one RNC (“Radio Network Controller”) and one or morelogical nodes known as “Node Bs”. The RNC provides functionality similarto the base station controller in GSM networks and each Node B is theaccess point of a cell, equivalent to the base station in GSM networks.The RNS, and in particular the RNC, provides the following functions:

-   -   radio resource control (“RRC”)    -   admission control    -   congestion control    -   power control    -   handover control

The core network has three primary domains, these being acircuit-switched domain, a packet-switched domain and the register andservice domain. It provides three main functions:

-   -   connection management, comprising the provision of bearer and        other services for circuit-switched connections    -   session management, comprising the set-up, release and        monitoring of packet-switched connections    -   mobility management, comprising the monitoring of UE location        and maintenance for example of the HLR (“Home Location        Register”)

In a cellular communication environment, several different UMTS networksmay be available at the same geographical location. In UMTS terminology,these are known as PLMNs (“Public Land Mobile Networks”). In use, a UEis usually configured to make connection to a preferred PLMN at any onelocation.

A UE in a cell of a PLMN can be in one of at least three differentoverall states: detached; idle; and connected.

In the detached state, the UE is switched off and there is nocommunication with it and no need for the network to track its location.

When the UE is switched on, it goes to idle mode. A PLMN is selected andthe UE searches for a suitable cell to “camp on”. In practice, the UEcan camp on a UTRAN, GSM or GPRS cell. In the context of embodiments ofthe present invention however, GSM or GPRS cells are only of limitedinterest.

Camping on a cell means that the UE has chosen that cell to provideavailable services. This cell is known as the serving cell. It hascompleted a cell selection or reselection process and it tunes to thebroadcast control channel (“BCCH”) of the serving cell to receivebroadcast system information. System information is broadcast in blocksof predefined types. Once the UE is camped on a cell, it can receivesystem information blocks (“SIBs”) from the PLMN, including registrationarea information. It attempts to register using the non access stratum(“NAS”: protocols between a UE and the core network that are notterminated in the UTRAN). If successful, the UE can also receive pagingand notification messages from the PLMN and initiate call setup foroutgoing calls or other actions from the UE. This is known as beingcamped normally. If unsuccessful, the UE goes to the “any cellselection” state and thence to “camped on any cell” state. These statesin idle mode might arise for example if no suitable cell can be found inthe selected PLMN or there is no UICC inserted in the UE. Once camped onany cell, the UE may still be able to obtain limited service such asemergency calls only.

From idle mode, to change to connected mode, the UE sends a connectionrequest to the radio resource control function of the UTRAN. This iscalled a “RRC CONNECTION REQUEST” message. The UE then looks for aforward access channel (“FACH”) which will be used by the UTRAN to senda “RRC CONNECTION SETUP” message. There are four RRC connected statesavailable to the UE, as follows:

CELL_DCH State:

A dedicated physical channel is allocated to the UE in uplink anddownlink. The UE is known on cell level according to its current activeset (further discussed below). Dedicated transport channels and downlinkand uplink shared transport channels become available and a combinationof these transport channels can be used by the UE.

CELL_FACH State:

No dedicated physical channel is allocated to the UE. The UE camps on aserving cell and continuously monitors a FACH in the downlink. The UE isassigned a default common or shared transport channel in the uplink, forexample a random access channel (“RACH”), that it can use at any timeaccording to the access procedure for that transport channel. Theposition of the UE is known by the UTRAN on cell level according to thecell where the UE last made a cell update.

CELL_PCH State:

No dedicated physical channel is allocated to the UE. The UE camps on aserving cell and selects and monitors a paging channel, using anassociated paging indication channel (“PICH”). No uplink activity ispossible. The position of the UE is known by the UTRAN on cell levelaccording to the cell where the UE last made a cell update in CELL_FACHstate.

URA_PCH State:

No dedicated physical channel is allocated to the UE. The UE camps on aserving cell and selects and monitors a paging channel, using anassociated PICH. No uplink activity is possible. The location of the UEis known on UTRAN Registration Area (“URA”) level according to the URAassigned to the UE during the last URA update in CELL_FACH state.

A UE in CELL_DCH state may have, instead of a single cell, an active setof cells involved in a connection. This arises as follows. In practice,the radio transmission/reception to and from a connected UE iscontrolled via a Node B. However, a UE in a connected state can beconnected via several cells and their “Node B”s. These are called theactive set. A RNC combines the data streams from the different cells ofthe active set. To maintain connectivity, the UE measures the receivedsignal levels from the cells of its active set and reports themeasurements to the RNC. If there is a need for change, for examplebecause the UE has moved, the RNC instructs the UE to change its activeset.

A UE which is camped on a cell, for example in Cell_FACH, Cell_PCH, andURA_PCH connected states or in idle mode, has no active set. It hasinstead a serving cell from which it can obtain information.

Technical specifications published on the Internet by 3GPP which arerelevant to embodiments of the present invention include:

1) TS 25.304 (for example v3.13.0)

“User Equipment (UE) procedures in idle mode and procedures for cellreselection in connected mode”; and

2) TS 25.331 (for example v3.17.0)

“Radio Resource Control (RRC); protocol specification”

A UE moves between idle and connected modes and between differentconnected mode states. The 3GPP technical specifications mentioned abovedeal with cell selection in this type of circumstance as follows:

In TS 25.304 it is specified in Section 5.2.7 that when returning toidle mode from connected mode, the UE must select and camp on a suitablecell. Section 5.2.7.1 of TS 25.304 describes cell selection when leavingconnected mode to move to idle mode as “Candidate cells for thisselection are the cell(s) used immediately before leaving connectedmode.” That is, the existing serving cell or active set. It is notspecified what to do in the case of exiting one connected state such asCell_DCH and moving to another connected state, such as Cell_FACH,Cell_PCH, or URA_PCH.

Section 5.4.4 of TS 25.304 states that the UE will only perform cellselection in connected mode in the case of Out of Service area and thecell selection process then used is the same as that used for idle mode.This is described in TS 25.304 Sections 5.2.3 and 5.2.7 (though 5.4.4specifically refers to 5.2.3). Relevant radio bearer control proceduresare set out in TS 25.331 v3.17.0, Sections 8.2.2.2 (in particular NOTE 3in that section), 8.2.2.3, and 8.5.2. Selecting a cell is however alsorequired when changing frequency when in Cell_FACH, as specified in8.2.2.3.

The following types of transition are of particular interest:

a) Cell_DCH to Cell_FACH, Cell_PCH, or URA_PCH;

b) Cell_FACH to Cell_FACH, Cell_PCH, or URA_PCH; and

c) Cell_FACH or Cell_DCH to Idle

Clearly from TS 25.331, cell selection is also required in the case a)where the UE moves out of Cell_DCH and remains in connected mode. Thatcase is not covered by TS 25.304, despite TS 25.331 referencing thatdocument for that functionality. Another factor is raised in relation tocase a) by Section 8.2.2.3 of TS 25.331. Here it is stated that if thecell that the UE selects is not the cell indicated by the UTRAN in themessage that caused the UE to leave Cell_DCH, then the UE should performa Cell Update procedure. Hence if the cell indicated by the UTRAN is notin the active set for the UE prior to leaving Cell_DCH, in which case itcannot be selected, the UE will have to perform a cell updateimmediately after selecting one of the cells from the active set. The UEmay then perform cell reselection to the UTRAN specified cell if (a) itis the best cell, and (b) it is in the list of neighbouring cells forthe selected cell.

Case b) only requires cell selection if a frequency is specified, asdetailed in 25.331 section 8.2.2.3.

Selecting a cell for case (c) is covered in section 5.2.7 of TS 25.304.However, the Cell_FACH transition to Idle case only requires cellselection if a different frequency can be specified, which has beenproposed as a change request to TS 25.331.

So in the prior art, upon transition between connected states or from aconnected state to idle mode, the only applicable cell selectionprocedures select a cell from an existing active set for a UE or use theexisting serving cell. However, this can result in inefficient use ofresources. The serving cell may not be the best available cell and theactive set may not include the best available cell from which to select.The serving cell or active set was selected on the basis of the capacityneeds of the UE in one particular connected state. Once the UE movesfrom that state, the serving cell or active set may no longer be themost appropriate. Efficiency of the radio link and power consumption mayno longer be optimised in a different state since the communicationneeds of the UE will have changed.

Further, the serving cell may not be, and the active set may not haveincluded, a cell indicated by the UTRAN at the time of leaving aconnected state. The UTRAN may have indicated a cell based on morerecent data, such as congestion data, than the data available at thetime of selecting the existing serving cell or active set.

According to a first aspect of embodiments of the present invention,there is provided cell selection apparatus for use in mobilecommunications equipment to select a cell from a candidate set of cellsto support subsequent communication between a cellular network and theequipment, said apparatus being triggerable, by a transition of theequipment out of an existing connected mode state in which communicationis supported by a serving cell or an active set of one or more cells, toselect said cell,

wherein the apparatus is adapted to use as said candidate set a set ofcells comprising at least one cell which is not a cell supporting theexisting connected mode state, where said at least one cell meets atleast one predetermined criterion.

Said transition of the equipment might be either to a new connected modestate or to idle mode.

It will be understood that it is not necessary for the transitionalready to have occurred to trigger said apparatus. A triggering eventmay for example be receipt of a message or signal by the mobilecommunications equipment or the cell selection apparatus that atransition is to take place.

Embodiments of the invention are relevant where the cellular networkcomprises a UMTS network and the candidate set of cells comprises UTRANcells. However, there may be circumstances, such as radio link failurein Cell_DCH where the UE cannot find any UMTS cells, where the candidateset of cells may be enhanced by the addition of one or more GSM cells.

The mobile communications equipment may comprise a UE as referred to inTS 25.304 and TS 25.331.

Although the apparatus is adapted to use a candidate set of cells inwhich at least one cell is not a cell supporting the existing connectedmode state, it may be the case that there is no cell meeting the atleast one predetermined criterion. There may be occasions when theserving cell or active set provides an adequate candidate set. However,by being capable of using a candidate set of cells which goes beyond theexisting serving cell or one or more cells of the active set, theapparatus can provide significant improvements in cell selection inseveral situations.

In a first example, the transition of the equipment from an existingconnected mode state could have been in response to a UTRAN messageindicating a UTRAN preferred cell. In the prior art, if the UTRANpreferred cell is not amongst the one or more cells supporting theexisting connected mode state (that is, the UTRAN preferred cell is notthe existing serving cell or in the existing active set), then it willnot be selected. However, the UTRAN is likely to have significantreasons for preferring a cell, such as congestion avoidance in othercells or to reduce the need for cell updates by the UE by choosing alarge cell. If the UTRAN preferred cell is not selected, the UE isimmediately forced to perform a cell update procedure, for example asspecified in Section 8.2.2.3 of TS 25.331.

Thus the at least one predetermined criterion might be that said atleast one cell comprises a cell identified to the mobile communicationsequipment by the network. Such a cell could be a UTRAN preferred cell ora cell of a virtual active set.

(A virtual active set is a set which has been enhanced by the UTRAN toinclude inter-frequency cells as well as intra-frequency cells.)

In a second example, the existing serving cell or active set may have astrong but neighbouring cell. If this is the case, a cell updateprocedure is likely to be run as soon as permitted as the UE becomesaware of the strong neighbouring cell.

Thus it may be preferred that in the cell selection apparatus describedabove, said at least one further cell comprises a cell neighbouring theserving cell or a cell of the active set. In this case, the at least onepredetermined criterion might be that said at least one cell has astrong power measurement associated with it. This might be a storedpower measurement or for instance one taken at the time of thetransition.

In an alternative arrangement, said at least one further cell maycomprise a cell identified by historic cell-related information arisingfrom past behaviour of the communications equipment, such informationbeing stored by or in relation to the mobile communications equipment.For example, the information might indicate that a cell had often orrecently been selected, visited or measured by the equipment in thepast. In this case, the at least one predetermined criterion might be ameasure of that behaviour, such as number of times or period of time.

It is known to use power measurements in cell selection. However, thisis not the only possible factor in selecting cells. Various selectioncriteria could be used, being based on at least one parameter for eachof the candidate set of cells. However, it may be preferred to givepreference to a type of cell, for instance a cell in the candidate listwhich is a network-preferred cell. To implement this, the mobilecommunications equipment might further comprise parameter adjustmentmeans for preferentially adjusting at least one parameter for at leastone of the candidate set of cells, thereby changing the probability thatsaid at least one cell will be selected.

According to a second aspect of embodiments of the present invention,there is provided a method of selecting a cell of a cellular network tosupport subsequent communications between the network and mobilecommunications equipment on transition of the equipment out of anexisting connected mode state in which communication is supported by aserving cell or an active set of one or more cells, which methodcomprises the steps of:

i) assembling a candidate list of cells; and

ii) selecting a cell from the candidate list,

wherein step i) comprises identifying for said candidate list at leastone cell which is not a cell supporting the existing connected modestate, said at least one cell meeting one or more predeterminedcriteria.

The one or more predetermined criteria might be as described above inrelation to the first aspect of the invention.

Further inventive features of embodiments of the invention are as setout in the claims hereto.

It should be noted that any feature described in relation to one aspectof the invention may be applied in relation to one or more other aspectsof the invention if appropriate. Features may indeed be applied in anycombination without departing from an embodiment of the invention.

Cell selection apparatus and methods according to embodiments of thepresent invention will now be described, by way of example only, withreference to the following figures in which:

FIG. 1 shows a schematic block diagram of a UMTS cellular network inwhich the cell selection apparatus and methods might be applied;

FIG. 2 shows a schematic block diagram of a UE for use with the networkof FIG. 1; and

FIG. 3 shows a functional block diagram of components of the UE of FIG.2 with particular relevance to cell selection.

Referring to FIG. 1 and as described above, a UMTS cellular networkusually has two primary parts, the UTRAN (“UMTS Terrestrial Radio AccessNetwork”) 100 and the core network 105. The UTRAN 100 looks after thephysical aspects of providing wireless access for a mobile UE (not shownin FIG. 1) to the core network 105 and the core network providesswitching. The UTRAN and the core network communicate with each othervia an interface known as the “Iu” 110.

The UTRAN 100 is made up of a set of RNSs (“Radio Network Subsystems”).Each RNS comprises one RNC (“Radio Network Controller”) 115 and one ormore logical nodes known as “Node Bs” 120. The RNC 115 providesfunctionality similar to the base station controller in GSM networks andeach Node B 120 is the access point of a cell 125, equivalent to thebase station in GSM networks. The RNS, and in particular the RNC 115,provides the following functions:

-   -   radio resource control (“RRC”)    -   admission control    -   congestion control    -   power control    -   handover control

In embodiments of the present invention, these functions are of knowntype and as disclosed in the publications of bodies such as the 3GPP,including the technical specifications referenced above.

The cellular nature of the network is also of known type and maycomprise a hierarchical structure of cells 125 including macro cells toprovide geographical continuity of service, micro cells for highlypopulated but low mobility areas and pico cells, usually used for anindoor environment where there are many high capacity but low mobilitywireless UEs. Referring to FIG. 2, the UE 200 again is of generallyknown type and acts as the counterpart of the core network 105, the RNC115 and the Node B 120 in implementing cellular communications. However,the UE 200 is novel in its cell selection procedures.

The UE 200 as shown in FIG. 2 is a mobile telephone handset. However,many different forms of UE 200 already exist, such as personal digitalassistants and laptop computers, and embodiments of the presentinvention are not limited to any particular form of UE.

The UE 200 generally has a radio antenna 215 for communicating via aNode B 120 across a network in known manner and provides a generallyconventional user interface, for example a keypad 210 and display screen205, a loudspeaker 245 and a microphone 250. The UE also provides aprocessor 265 and operating environment 270 for running softwareprocesses 220 together with some, often limited, data storage capacity230.

Where the UE 200 is provided as a mobile telephone handset, it is alsoprovided with a smart card reader 275 of known type for interacting witha removable card such as a Universal Integrated Circuit Card (“UICC”)235. The UICC 235 is also provided with a processor 255 and operatingenvironment 260 and carries a data structure 225, and one or moresoftware applications 240. Nowadays, a UICC 235 is effectively acomputer, supporting for example user personalisation, securecommunications including encryption, and international roaming. However,it is usually the processor 265, operating environment 270 and softwareprocesses 220 of the UE 200 itself which run the functions concernedwith signalling to and from the cellular network.

The UE 200 interacts with the components of the cellular network inproviding functions such as the following:

Node B 120:

Encoding and interleaving

Power control

Radio measurement

Spreading and despreading

Modulation and demodulation

RNC 115:

Acknowledged mode

Radio resource control

Handover and cell selection

Ciphering and deciphering

Core Network 105:

Mobility management

Session management

Bearer negotiation and service requests

Embodiments of the present invention are primarily implemented in thesoftware processes 220 for interacting with the RNC 115 and particularlycell selection.

Transitions between idle mode and connected mode states may be triggeredfor various reasons, such as the UE 200 being switched on andsubsequently used to establish a voice or data connection or due tochanges in network conditions. The reasons for a transition are notgenerally important in an embodiment of the invention, only the responseof the UE 200 in cell selection.

As discussed above, known procedures of a UE 200 in response to atransition out of a connected mode state, either to idle mode or toanother connected mode state, are to perform a cell selection based on acandidate set of cells which is the existing serving cell or active setfor the UE 200 prior to transition. In a transition from Cell_DCH, wherethe candidate set of cells is the existing active set, the UE 200primarily makes the selection based on power measurements for signalsfrom the Node Bs 120 of the active set of cells. These powermeasurements can be made and updated by the UE 200 itself.

In embodiments of the present invention, the UE 200 performs a cellselection in response to a transition out of a connected mode state byfirst applying a selection algorithm to select a candidate set of cellsand only then performing cell selection. The step of selecting thecandidate set of cells may comprise for example taking the existingserving cell or active set, adding a cell preferred by the UTRAN if notalready the existing serving cell or in the existing active set, and/oradding one or more cells neighbouring the existing serving cell or theexisting active set, and/or adding one or more cells identified byhistoric cell-related information such as information arising fromprevious behaviour of the UE 200.

Cell selection might then be performed in known manner based for exampleon power measurements. However, in a further innovative step, valuesassociated with a cell in the candidate set may be enhanced or alteredprior to cell selection. This enables for example a cell preferred bythe UTRAN to have increased likelihood of being selected, although theactual cell selection mechanism is of known type.

If the step of selecting the candidate set of cells is to comprise thepossible inclusion of one or more neighbouring cells into the candidateset, the UE 200 needs to have information identifying the relevantneighbouring cells. This can be obtained from system information. Systeminformation is broadcast within a cell and provides information aboutthe network configuration. It is broadcast in system information blocks(“SIBs”) which group together system information elements of the samenature. Different SIBs may have different characteristics, for exampleregarding their repetition rate and the requirements of the UE 200 toread the system information. In known arrangements, the information canenable a UE 200 to perform initial cell selection procedures. Typicalsystem information messages will include parameters such as networkidentification, neighbouring cells, channel availability and powercontrol requirements.

In the following, a set of situations is described in which prior artarrangements give rise to a less than optimum cell selection.

Situation 1

Unless a cell suggested by the UTRAN 100 is the existing serving cell orin the existing active set, or if it is relatively weak, the UE 200 maynot select it. There are three consequences of this behaviour. Firstlyan immediate cell update will be required, e.g. as specified in TS25.331 section 8.2.2.3. Secondly, the UE 200 may select a cell that theUTRAN 100 would prefer was not selected, e.g. because that cell iscongested. Thirdly there may be a reason why the UTRAN 100 specified aparticular cell. For example, it may have specified a large cell toreduce the need for the UE 200 to perform cell updates due to the UE 200moving.

The UE 200 selecting the cell suggested by the UTRAN 100 benefits theUTRAN 100 with regards to network utilisation and UE distribution andcontrol. Consider the following examples. While in CELL_DCH a monitoredset of cells can differ from that in CELL_FACH and idle mode. Thenetwork may be using a system of large, low capacity macro “umbrella”cells and high capacity micro cells. In CELL_DCH the UE 200 is connectedto one or more micro cells, which form its ‘active set’. When the UEmoves to CELL_FACH it is likely to select one of these micro cells tocamp on, but the UTRAN 100 would prefer that the UE 200 be camped on anumbrella cell.

If the UE 200 is required to try the UTRAN preferred cell first then theUE 200 would camp onto an umbrella cell, and from the updated list ofneighbour cells reselect between it and other umbrella cells.

Another example is when several networks are sharing resources. In thiscase the operator supporting the call would wish the UE 200 to camp ontoa cell that offered the most advantage to the operator.

Situation 2

The existing serving cell or active set of cells may exclude a strongcell. If the UE 200 first tries to select a cell from a candidate set ofcells that does not include a strong cell close by, there is a dangerthat a weaker cell will be selected. After one second when a cellreselection is allowed, a stronger neighbour cell of the selected cellmay be selected. Had that stronger cell been in the set of candidatecells, the cell reselection would not have been necessary, assuming cellstrengths remain constant. Even if cell strengths vary, it is stilllikely to reduce the number of cell reselections if the strongest cellis selected, all other things being equal.

Situation 3

The UE 200 may not find a suitable cell in its set of candidate cells.In that case, a wider cell search is required. That may take more timeand battery energy compared to using a better set of candidate cells.

In more detail, embodiments of the invention will find application asdescribed in the following scenarios. The behaviour of the UE 200 incell selection in these scenarios can overcome the difficultiesmentioned in the “Situations 1 to 3” above. It should be noted that thebehaviours of the scenarios described below are not mutually exclusiveand there may be advantage in applying more than one of them in a cellselection procedure.

Scenario 1: Adding a UTRAN Supplied Cell to a Candidate Set

By adding the cell specified by the UTRAN to the list of cells fromwhich a selection can be made, it is now possible for the UE to selectthat cell.

Scenario 2: Adding Strong Measured Cells to a Candidate Set

By adding strong cells already known to the UE 200 into the candidateset of cells, it reduces the chance of no cells being found, and itreduces the chance that a weaker cell will be selected, only to reselecta stronger cell one second later. In particular, if the UE 200 isleaving Cell_DCH because of a radio link failure, then this may bebecause all of the cells in the active set have become too weak to besuitable (although it is also possible for example that the dedicatedchannel(s) has been switched off and that the cells in the active setare still viable).

Scenario 3: Preferring a UTRAN Supplied Cell in a Candidate Set

In this scenario, preference might be given to the UTRAN supplied cell,if there is one. This might be done by ignoring the strength of othercells and selecting the UTRAN supplied cell as long as it meets certaincriteria. These criteria may be simply those for a cell to be a suitablecell (for instance as defined in UTRAN relevant standards material).Alternatively, an advantage can be given to the UTRAN supplied cell butthat cell is still compared against other cells. For example, Squal (thecell selection quality value referred to in TS 25.304) or Srxlev (thecell selection RX level value referred to in TS 25.304) or both could beadjusted for the UTRAN supplied cell, and then these values could becompared with values for other candidate cells. This solution has theadvantage that the UTRAN supplied cell is more likely to be selected,and so allows the benefits of that behaviour to be realised.

Scenario 4: Adding One or More Neighbour Cells to a Candidate Set

In this scenario, the set of candidate cells is augmented with cellsthat are specified to be neighbours of those cells. For example, in thecase of leaving Cell_DCH, if the active set contained the cells A and B,and cell A had neighbours B and C, and cell B had neighbours A and D,then the UE 200 could augment the set {A, B} to be {A, B, C, D}.Consider the case where D is stronger than B, and B is stronger than Aand C. If the UE 200 had not applied this solution, it would haveselected cell B, only to perform a cell reselection as soon as it could(one second later) to the stronger cell D. The solution ensures that Dis selected initially, and so no cell reselection is required.

This solution could be applied iteratively. For example, in the aboveexample if Cell E was a stronger neighbour of cell D, then cellreselection of cell E would occur. However, if the neighbours of theneighbours of the active set were also added to the set of candidatecells, then cell E would be selected in the first place. It is likelyhowever that for each iteration of adding in the neighbour cells, thereis less chance of finding a stronger cell. A single iteration of thistechnique is likely to be found optimal or near optimal.

An enhancement of the technique is as follows. Start with a candidateset X, add to X the neighbour cells of cells in X, and call this new setY. If the strongest cell in Y is in X, then none of its neighbours arestronger. But if the strongest cell in Y is cell c(1), which is not inX, then iterate the following process for i=1, 2, 3, etc. until the exitcondition is satisfied. Measure the neighbour cells of cell c(i). If noneighbours of cell c(i) are stronger than cell c(i), then select cellc(i) as the cell on which to camp. Otherwise, pick the strongestneighbour of cell c(i), and call this c(i+1), and repeat the process.Because cells have a finite set of values for strength, and c(i+1) isalways stronger than c(i), this process must terminate. It may bedesirable to terminate this process after a fixed number of iterationshowever, to speed up the procedure at the expense of possibly picking acell with a stronger neighbour.

In order to apply the behaviours of this scenario, information aboutneighbour cells is required. If the UE 200 is moving from Cell_DCH toanother connected mode (Cell_PCH, URA_PCH, or Cell_FACH) then SIB types11 and 12 define the neighbour cells for cell reselection. If the UE 200is moving from Cell_PCH, URA_PCH, or Cell_FACH to Idle mode then SIBtype 11 defines the neighbour cells for cell reselection. The UE 200could store these SIBs whenever it reads them. Even if they expirebecause six hours elapse since they were read, they could still be usedfor this purpose. Cell neighbour lists may not change much over time,and so using an old list may well be much better than not takingneighbours into account. As the UE 200 is likely to have limited memoryfor storing such data, choices will need to be made about which cellsinformation to overwrite.

If the UE does not have a stored neighbour list that could be used bythis technique, then it could either use an empty list, or it could readthe relevant SIB (SIB11 or SIB12) from the relevant cell. If the UE 200has identified that a cell c(i) is potentially the cell on which it willcamp, then it has to read some SIBs to ensure it is a suitable cell. Itmay be that while waiting for those SIBs, the relevant neighbouring cellSIB (i.e. SIB11 or SIB12) is scheduled. In that case it may be worthmeasuring any unmeasured cells to check if there is a strongerneighbour. However, if the UE 200 does not have neighbour informationfor several cells in the original set of candidate cells, it probablyisn't worth reading those SIBs before taking measurements. Besides, ifc(1) is the strongest cell, the neighbours of other cells in theoriginal set are not relevant.

Scenario 5: Miscellaneous Additions to Set of Candidate Cells

To increase the chance of the measurements on a candidate set of cellsresulting in detection of a suitable cell, various classes of cellscould be added to that set. The time taken to perform the measurementsis likely to increase with the size of the set, and there is acompromise between taking slightly longer and increasing the chances ofsuccess, which may save time because larger searches are not required.

Examples of classes of cells that could be added to the set of candidatecells are cells based on historic behaviour of the UE, as follows:

-   -   1. recently measured cells;    -   2. frequently measured cells;    -   3. recently visited cells;    -   4. frequently visited cells;    -   5. recently selected cells;    -   6. frequently selected cells; and    -   7. any cell which has been detected for a long period,        particularly if the UE has been moving about.

A cell in category “7.” is likely to be a large or reliable cell.

It may be the case that there is no UTRAN preferred cell but the UE 200may still be able to identify a suitable candidate cell from SIBs ofthat cell. For example, a cell may be an umbrella cell as describedabove, having low bandwidth but large geographical area. Such a cellmight be suitable for the UE 200 in its post-transition connected modestate or idle mode even if it was not suitable in the pre-transitionstate.

An example of another class of cell that could be added to the set ofcandidate cells are cells in a virtual active set.

The above scenarios can be generally described as cell selectionprocedures in apparatus for selecting a cell as a serving cell for userequipment within a wireless network when the equipment is transitioningfrom a connected mode state in which communication is provided by apre-transition serving cell or active set. The apparatus for putting theprocedures into effect will comprise cell selection apparatus, forinstance supporting a channel configuration mechanism, and listing meansfor configuring a list of candidate cells for selection by the selectionapparatus, wherein the listing means is adapted to configure a list thatcomprises at least one cell selected from the following:

-   i) one or more cells neighbouring the pre-transition serving cell or    neighbouring one or more cells of the pre-transition active set;-   ii) a cell specified by the network;-   iii) one or more cells identified by stored information of the    equipment; and-   iv) a cell identified by system information received by the mobile    communications equipment.

The cell selection apparatus may select a cell on the basis of whethercells in the candidate list meet at least one predetermined criterion.For example, said at least one predetermined criterion might be forinstance a relative power measurement and/or simply that said at leastone cell is a cell specified by the network. Power measurements for thecells in the candidate list might for example be obtained in real time,by power measurement apparatus of known type, or from stored data.

A cell specified by the network might for instance be specified as aUTRAN preferred cell at the time of making the transition or might be acell of a virtual active set.

The channel configuration mechanism may then comprise a cell selectionmechanism for selecting a cell from a candidate list as described above,attempting to tune to the broadcast control channel of the selectedcell, establishing said tuning if the attempt is successful, or if notsuccessful, repeating the process continuing with a different cell fromthe configured list of candidate cells. The different cell may suitablybe the one that has the greatest measured power.

Such a cell selection procedure, and the apparatus for putting it intoeffect, allows a broader group of cells from which to select than theprior art, thereby enabling mobile user equipment to pick a good cellfrom the start, avoiding inefficient use of radio resources and power.

Referring to FIG. 3, the components present in a UE 200 to enable anembodiment of the present invention may generally be provided inassociation with channel configuration apparatus 300 installed amongstthe software processes 220 shown in FIG. 2. The channel configurationapparatus 300 has access to the data storage capacity 230 of the UE 200and it receives three general types of input 325, 330, 340 these beingstate transitions 325 originated by the UE 200, network communications330 and power measurements 340. These inputs can be used as appropriateby cell selection apparatus 305, in the event of a transition from aconnected mode state to another connected mode state or idle mode, toselect and output a new serving cell 320. The channel configurationapparatus 300 will then tune to an appropriate channel of the newserving cell, such as the broadcasting control channel, for receivingsubsequent communications at the UE 200 from the network. In the event anew serving cell cannot be selected, it may alternatively be necessaryto instigate a cell search.

The network communications 330 might comprise SIBs which as describedearlier can for instance be used to obtain lists of neighbouring cells.This network-supplied data can be stored by the data storage processes315. However, there are other network communications which are relevantto the cell selection apparatus 305, these being for example:

-   -   1. Reconfiguration messages (which may contain a UTRAN-specified        cell)    -   2. Cell Update Confirm messages (which may contain a        UTRAN-specified cell)    -   3. RRC Connection Release messages (which force the UE to idle        mode)

In order to provide appropriate tuning, the channel configurationapparatus 300 has the following components:

-   -   cell selection apparatus 305 for applying selection algorithms        and including a parameter adjustment mechanism for favouring        selection of for example a UTRAN preferred cell    -   power measurement equipment 310 for measuring the power of radio        cells (in known manner)    -   data storage 230 and processes 315 for storing network-supplied        data, historic cell-related information arising from past        behaviour of the UE 200, cell power measurements and selection        algorithms

In a process provided by the channel configuration apparatus 300, thefollowing steps may be taken in response to an input indicating aconnected mode state transition:

STEP 1: an overall cell selection process 305 starts to run. This mustassemble a candidate set of cells for making a selection. It does thisaccording to a selection algorithm by doing one or more of thefollowing:

-   i) listing an existing active set or serving cell of one or more    cells for the UE 200 prior to transition,-   ii) adding any UTRAN preferred cell or cell in a virtual active set    which may have been notified in an incoming network communication    330,-   iii) referring to stored SIB data to identify cells neighbouring the    cells of the existing active set or serving cell,-   iv) referring to stored cell power measurements, and-   v) referring to stored (historic) cell-related information arising    from past behaviour of the UE 200 to identify cells from the list 1.    to 6. under “Scenario 5” above.

The strategy for assembling a candidate list of cells from thecategories above in any particular circumstance can be set in the UE200, for example as an updatable stored selection algorithm. Such analgorithm may be relatively simple, such as adding cells from thecategories i) to v) above in sequence until the candidate list hasreached a threshold number of cells in it. Alternatively, an algorithmcould set a threshold in relation to each category, such as using a cellfrom category iv) only if the stored power measurements indicate thecell had a power significantly greater, say 150%, of the power of anycell in the existing active set or of the existing serving cell.

STEP 2: having assembled a candidate list, the cell selection process305 obtains power measurements in respect of the cells, the measurementsbeing obtained in known manner by the power measurement equipment 310and/or by reference to stored data.

STEP 3: the cell selection process 305 uses its parameter adjustmentcapability to adjust a value for at least one parameter for one or morecells in the candidate list, thus changing its chances of selection.This is likely to be done in particular so as to enhance the chances ofany UTRAN preferred cell to be selected. However it could also be donefor example to decrease the value of stored power measurements inrelation to measurements obtained in real time. Parameter adjustmentdata, such as rules for carrying it out, could be pre-programmed in thecell selection apparatus 305 or could be delivered to it in networkcommunications such as SIBs.STEP 4: the cell selection process 305 now selects from the cells in itscandidate list to identify a new serving cell in an output 335 for usein channel configuration between the UE 200 and the network. This can bebased on one or more parameters for the cells but might simply be basedon the power measurements, after adjustment. In the event a new servingcell cannot be selected, it may alternatively be necessary to instigatea cell search.

Although called a process, the cell selection process 305 can equally beviewed as a “candidate set assembler”: apparatus which in use generatesan output, a selected serving cell, which subsequently controlsbehaviour of the UE 200 in configuring its communications.

It might be noted that the word “comprising” as used in thisspecification is intended to be broadly interpreted so as to include forinstance at least the meaning of either of the following phrases:“consisting solely of” and “including amongst other things”.

The invention claimed is:
 1. A mobile device for use in a wirelessnetwork, the device comprising cell selection apparatus for use toselect a cell from a candidate set of cells to support subsequentcommunication between a wireless network and the device, said cellselection apparatus being triggerable, by a transition of the device outof an existing connected mode state in which communication is supportedby a serving cell or an active set of one or more cells, to select saidcell, wherein the mobile device comprises a data store for storing cellpower measurement data; wherein the cell selection apparatus is adaptedto use as said candidate set a set of cells comprising at least one cellwhich is not a cell supporting the existing connected mode state andwherein the cell selection apparatus further comprises parameteradjustment means arrange to adjust at least one parameter for at leastone cell of the candidate set of cells, thereby changing a probabilitythat said at least one cell will be selected, the parameter adjustmentbeing performed at the mobile device, wherein said parameter adjustmentmeans is arranged to adjust a stored value of a previously measuredpower measurement for the at least one cell.
 2. The mobile deviceaccording to claim 1 wherein said at least one cell comprises a cellidentified to the mobile device by the network.
 3. The mobile deviceaccording to claim 1 wherein said at least one cell comprises a cellneighboring a cell supporting the existing connected mode state.
 4. Themobile device according to claim 1 wherein said at least one cellcomprises a cell identified by information stored by the mobile device.5. The mobile device according to claim 4 wherein said informationstored comprises historic cell-related information arising from pastbehaviour of the mobile device.
 6. The mobile device according to claim4 wherein said information stored concerns power measurement data inrelation to said at least one cell.
 7. The mobile device according toclaim 1 comprised by channel configuration apparatus in the mobiledevice for configuring a communication channel in the network.
 8. Themobile device according to claim 1 wherein said parameter adjustmentmeans is adapted to select a cell identified to the mobile device by thenetwork for said adjustment.
 9. The mobile device according to claim 1,further comprising power measurement equipment for taking powermeasurements with respect to cells of the candidate set of cells,wherein the selection of a cell is based at least in part on said powermeasurements.
 10. The mobile device according to claim 1, furthercomprising an algorithm store for storing at least one selectionalgorithm for use in assembling a candidate set of cells.
 11. The mobiledevice according to claim 1, further comprising a data store for storingsystem information for use in identifying one or more cells neighbouringa cell supporting the existing connected mode state.
 12. The mobiledevice according to claim 1, further comprising a data store for storingdata relevant to historic cell-related information arising from pastbehaviour of the mobile device.
 13. The mobile device according to claim1, wherein the network comprises a UMTS-based network.
 14. The mobiledevice according to claim 1 wherein said transition of the devicecomprises a transition from a state in which it has a dedicated channelto a state in which it is camped on a cell.
 15. The mobile deviceaccording to claim 13 wherein said existing connected mode statecomprises any one of Cell_DCH, Cell_FACH, Cell_PCH, and URA_PCH.
 16. Themobile device according to claim 1 wherein said transition of the deviceout of an existing connected mode state comprises a transition to anidle mode.
 17. The mobile device according to claim 13 wherein saidtransition of the device out of an existing connected mode statecomprises one of the following transitions: a) Cell_DCH to Cell_FACH,Cell_PCH, or URA_PCH; b) Cell_FACH to Cell_FACH, Cell_PCH, or URA_PCH;and c) Cell_FACH or Cell_DCH to an idle mode.
 18. A method of a mobiledevice selecting a cell of a network to support subsequentcommunications between the network and the mobile device on transitionof the mobile device out of an existing connected mode state in whichcommunication is supported by a serving cell or an active set of one ormore cells, which method comprises the mobile device: i) assembling acandidate list of cells; and ii) selecting a cell from the candidatelist, wherein operation i) comprises the mobile device identifying forsaid candidate list at least one cell which is not a cell supporting theexisting connected mode state, said at least one cell meeting one ormore predetermined criteria; and further comprising the mobile deviceadjusting a value for at least one parameter of a cell in the candidatelist, prior to operation ii), so as to change the likelihood ofselection of said cell, the parameter being performed by the mobiledevice, wherein said at least one parameter comprises a stored value ofa previously measured power measurement for the cell.
 19. A methodaccording to claim 18 which further comprises the operation of receivingan input identifying a transition from the existing connected mode stateof the mobile device.
 20. A method according to claim 18 which furthercomprises the operation of receiving data identifying anetwork-preferred cell, and wherein the at least one cell comprises saidnetwork-preferred cell.
 21. A method according to claim 18 wherein saidoperation of adjusting a value comprises adjusting a value for at leastone parameter of the network-preferred cell.
 22. A method according toclaim 18, further comprising the operation of storing system informationfor use in identifying said at least one cell.
 23. A method according toclaim 18 which further comprises the operation of storing historiccell-related information arising from past behaviour of the mobiledevice, for use in identifying said at least one cell.
 24. A methodaccording to claim 18 which further comprises the operation of storingcell power measurement data, for use in identifying said at least onecell.
 25. A method according to claim 18 wherein operation ii) iscarried out by reference to power measurements for the cells of thecandidate list.